Digitally controlled variable frequency HF emphasis circuit for use in video displays

Abstract

A sharpness filter processes a video signal that is to be displayed in a windowed environment. The windowed environment may contain windows that for displaying television images derived from video standards such as NTSC, PAL, and SECAM. Displaying windowed television images results in line scan rates and spatial frequencies that differ from the line scan rate and spatial frequency specified by the original video standard. The sharpness filter of the invention emphasizes certain high frequency components of the video signal in accordance with the encountered spatial frequencies. A control circuit for the sharpness filter provides a linear correlation between a user control and the center frequency of the sharpness filter. The sharpness filter shapes the output signal to avoid saturation and overdriving of video input amplifiers. The circuit can be implemented using a low-cost CMOS process.

1. A method for providing an enhanced video image signal from a video input signal, comprising:
 inverting the video input signal to provide an inverted video input signal; 
 coupling the video input signal to a first high pass filter, wherein the first high pass filter has a first pass band that is defined by a first corner frequency; 
 providing a first filtered signal with the first high pass filter, wherein the first filtered signal is generated in response to the video input signal; 
 coupling the inverted video input signal to a second high pass filter, wherein the second high pass filter has a second pass band that is defined by a second corner frequency that is different from the first corner frequency; 
 providing a second filtered signal with the second high pass filter, wherein the second filtered signal is generated in response to the inverted video input signal; and 
 combining the first and second filtered signals to produce a filtered video output signal, whereby an image produced by the filtered video output signal is sharpened. 
 
   
   
     2. The method of  claim 1  wherein the first video input signal is encoded according to a video standard that corresponds to at least one of NTSC, PAL, and SECAM. 
   
   
     3. The method of  claim 1  wherein the frequencies of the first and second pass limits are higher in frequency than the chrominance subcarrier frequency of the first video input signal. 
   
   
     4. The method of  claim 1  wherein the second corner frequency is lower than the first corner frequency. 
   
   
     5. The method of  claim 1  wherein the second corner frequency is lower than the first corner frequency by a factor of two. 
   
   
     6. The method of  claim 1 , further comprising: selecting the first corner frequency in response to a control code. 
   
   
     7. The method of  claim 6 , further comprising: selecting the second corner frequency in response to another control code, wherein the control code and the other control code are different from one another. 
   
   
     8. The method of  claim 1 , further comprising: selecting the second corner frequency in response to a control code. 
   
   
     9. The method of  claim 1 , further comprising: selecting the first corner frequency and the second corner frequency in response to a control code. 
   
   
     10. The method of  claim 1 , further comprising: combining the filtered video output signal with the video input signal to provide a video output signal. 
   
   
     11. The method of  claim 10 , wherein a first percentage of the filtered video output signal in the video output signal and a second percentage of the video input signal in the video output signal are specified by a mixer control code. 
   
   
     12. The method of  claim 1 , further comprising: displaying the filtered video output signal upon a display screen of a display monitor, wherein the video input signal is arranged for processing by a display device having a first line scan rate (scan 1 ) that is different from another line scan rate (scan 2 ) that is associated with the display monitor. 
   
   
     13. The method of  claim 12 , wherein the first corner frequency (fc 1 ) is determined by fc 1 =fchroma*(scan 1 /scan 2 ), wherein fchroma corresponds to a subcarrier frequency that is associated with the chrominance portion of the video input signal. 
   
   
     14. An apparatus for providing an enhanced video image signal from a video input signal, comprising:
 an inverting amplifier that is arranged to provide an inverted video input signal in response to the video input signal; 
 a first high pass filter that is arranged to provide a first filtered signal in response to the video input signal, wherein the first high pass filter has a first RC time constant; 
 a second high pass filter that is arranged to provide a second filtered signal in response to the inverted video input signal, wherein the second high pass filter has a second RC time constant; and 
 a summing junction that is arranged to combine the first and second filtered signals to produce a filtered video output signal, whereby an image produced by the filtered video output signal is sharpened. 
 
   
   
     15. The apparatus of  claim 14 , wherein the second RC time constant is greater that the first RC time constant. 
   
   
     16. The apparatus of  claim 14 , wherein the second RC time constant is greater that the first RC time constant by a factor of two. 
   
   
     17. The apparatus of  claim 14  wherein the first RC time constant is selected in response to a control signal. 
   
   
     18. The apparatus of  claim 14 , each of the first and second high pass filters further comprising: a programmable binary-weighted resistor network that is arranged to program a respective one of the first and second RC time constants in response to a corresponding control signal. 
   
   
     19. An apparatus for providing an enhanced video image signal from a video input signal, comprising:
 a means for inverting that is arranged to provide an inverted video input signal in response to the video input signal; 
 a first means for high-pass filtering that is arranged to provide a first filtered signal in response to the video input signal, wherein the first means for high-pass filtering has a first pass band that is defined by a first corner frequency; 
 a second means for high-pass filtering that is arranged to provide a second filtered signal in response to the inverted video input signal, wherein the second means for high-pass filtering has a second pass band that is defined by a second corner frequency; and 
 a means for combining that is arranged to combine the first and second filtered signals to produce a filtered video output signal, whereby an image produced by the filtered video output signal is sharpened. 
 
   
   
     20. The apparatus of  claim 19 , further comprising a means for selecting that is arranged to select the first and second corner frequencies in response to a control code.